This invention relates to a parameter sensing system employed in a fluid flowmeter compensating system which responds to a metered output of a flowmeter to provide a compensated metered output relating to a preselected reference parameter for the fluid.
A large number of temperature compensating systems have been developed to temperature correct flowmeters such as, for example, translating a fluid flow pulse signal generated by a volume flowmeter or turbine meter at an existing or ambient temperature into an equivalent flow signal at a predetermined base or standard temperature. Such temperature corrective flow measurement systems are generally classified in the United States Patent Office in CLASS 73, MEASURING AND TESTING, SUB-CLASS 194, VOLUME OR RATE OF FLOW METER and in CLASS 235, REGISTERS, SUB-CLASS 151.34, CALCULATORS.
In the custody transfer of fluid products such as liquid petroleum, for example, it is important to accurately determine the volume of the transferred product at some standard reference parameter such as temperature, because the volume of an essentially non-compressible or moderately compressible product such as liquid petroleum increases as the product temperature increases. Thus the product volume, such as indicated by a metering device, must be corrected to the volume that the product would have at the reference temperature because the product in most instances will not be metered at the reference temperature. The reference temperature used for such custody transfers is usually 60.degree. F in the United States, 15.degree. C in Europe, and 68.degree. F (20.degree. C) in South America.
A number of temperature compensators or temperature compensating totalizers have been developed to respond to a metered output in the form of electrical pulses supplied from a metered transducer and to temperature information from a temperature sensing element immersed in the fluid flow near the meter. Such compensators generally correct such flow indicative pulses to provide a series of output pulses representing the volume that the product would have at the reference temperature which are counted and recorded. A number of compensating systems have employed an analog to digital converter responding to sensed temperature variations at a temperature probe through a signal converting circuit to provide a digital output to a modification circuit which, in turn, modifies the metered volumetric flow indicative pulses, such as in U.S. Pat. Nos. 3,905,229; 3,895,529; 3,862,405 and 3,823,310.
A number of errors may occur in providing an output signal to be used in temperature compensating and indicative of the difference between the actual temperature of the fluid and the reference temperature. For example, most temperature sensing probes display non-linearity in their resistance change with respect to temperature. In many situations, the signal converting circuitry is remotely located from the sensing probe thereby necessitating the employment of lengthy inter-connecting circuits. Variations in impedance in the leads within such inter-connecting circuits or cables can result in distorted temperature output signals. Leakage resistance occurring between cable leads or direct short circuits between the leads or to ground or an open circuited lead can also result in a distorted and improper temperature output to the modification circuitry. Ambient conditions surrounding the cable leads or an improper circuit connection to the temperature probe may also vary the temperature output signal to provide an erroneous input to the compensating system. Errors may also occur in the signal converting and connecting circuitry due to impedance changes of elements because of component aging or the improper calibration of the signal converting circuitry to conform to the non-linearity between changes in fluid temperature and corresponding changes in probe resistance.
Additional errors may occur in the parameter sensing circuitry through variations in an energizing input. In addition, many temperature sensitive probes display differing resistivity characteristics at high temperatures as distinguished from lower temperatures which may also produce output errors where the signal converting circuitry does not distinguish between various ranges of temperature sensing and appropriately respond.
An improper operation may be overlooked in a situation where a malfunction in the temperature sensing and/or signal converting circuit has occurred resulting in the metered output being uncompensated or improperly compensated.